If the pacing rate of a pacemaker is maintained constant, the patient could not react in an optimum manner during both rest periods and effort periods, i.e., where the patient's level of activity is greater than at rest. Rate adaptive pacemakers typically modify the pacing rate, between a programmed maximum rate and a programmed minimum rate, based on a measure of a physiological parameter of the patient that is related to the patient s activity level. Such suitable physiological parameters that provide a measure of the physical activity of the patient include, e.g., blood temperature, cardiac output, the QT interval, the right ventricle oxygen saturation, respiration rate, and minute volume (also known as minute ventilation).
The minute volume is a physiological parameter that is known to vary linearly with the consumption of oxygen and therefore with the heart rate. The pacing rate (FC) is thus calculated by the pacemaker in accordance with a control function that relates the measure of minute volume (VE) and pacing rate FC via the linear relation: FC=a(VE)+b.
When initializing a rate adaptive pacemaker, it is necessary to provide coefficient values for the control function, for example, coefficients a and b of a linear function. In a usual manner, the coefficients are defined when implanting the pacemaker in the patient based on effort tests carried out in a hospital.
Document EP-0 151 689 refers to a pacemaker that uses the minute volume as representative of the activity of the patient and a straight line, linear control function relating the pacing rate and the minute volume. The straight line has a slope that is programmed by the physician based on an assessment of the patient's condition. Thus, the heart pacing rate varies between a programmed minimum pacing rate and a programmed maximum pacing rate as a function of the variations of the sensed minute volume according to the selected slope.
Document EP-0 299 208 refers to a pacemaker of this type in which the straight line representative of the pacing rate as a function of the signal received by the sensor of a physiological parameter is defined by means of two effort tests of the patient. The two tests are performed under different activity conditions, e.g., at rest and at a maximum effort level. These tests define the slope corresponding to the maximum minute volume and maximum pacing rate and the minute volume measure at rest and minimum pacing rate.
One problem with these techniques is that the control function is essentially fixed at implantation and does not allow for variations in the range of the patient's activity levels as in sleep, the starting again of an activity, and the practice of a sport. Another problem is that initialization is based on a medical assessment or controlled effort tests that are not representative of normal life. Consequently, the pacemaker may not provide a pacing rate to support all of the patient's activity. A further problem is that the function can be modified only by a further intervention of the physician and reprogramming of the pacemaker, e.g., in response to additional tests.
There is thus a continuing need to provide for improved rate adaptive pacemakers for pacing the heart as a function of a monitored physiological parameter. There is a further need for modifying the control functions relating the physiological parameter to the pacing rate based on recent patient activity without requiring intervention by the physician.